1. Field of the Invention
This invention relates generally to methods and devices for transferring molten metal from one vessel to another, and more particularly, to a method and device for shrouding the flow of molten metal from atmospheric gases during the transfer thereof from one vessel to another.
2. Description of the Invention Background
In processes for making steel and other alloys, metals are typically heated to a molten state and transferred from one vessel to another. In particular, the molten metal is often transferred or poured from a ladle into ingot molds or other vessels. The operation of transferring the molten metal is called teeming and the stream of molten metal is referred to as a teem stream.
The ladle, a large refractory-lined vessel which is used to receive and contain molten metal when it is removed from a furnace, serves to transport the molten metal to other locations in a steel or alloy making facility. The molten metal is transferred into other vessels whose character depend on the next processing step for the molten metal. It should be appreciated that ladles take different forms and shapes, and that the molten metal may be transferred out of the ladle by different methods. One common method for transferring the molten metal out of a ladle is called the bottom pour technique. In the bottom pour technique, the ladle includes a hole located on the bottom of the ladle which is sealed with a plug. The plug can then be removed by an actuator to permit the molten metal to flow from the ladle by gravity.
Usually, the molten metal is transferred from the ladle to some form of mold where the molten metal is allowed to cool and solidify. Typically, the molten metal is not transferred directly into the mold; instead it is transferred through a conduit into the mold. Often, the molten metal is poured into a funnel shaped, vertical tube or trumpet stand from which the molten metal is channeled into one or more molds.
A problem encountered in prior practice arose during the transfer of molten metal from the ladle to another vessel. Various gases from the ambient atmosphere in which the transfer occurs can be entrained in the molten stream. In particular, the molten metal may become contaminated with moisture or with gases such as oxygen, hydrogen or nitrogen. Contamination of the molten metal may lead to the reoxidation of the molten metal. The chemistry of the molten metal is carefully controlled to achieve specific properties in the alloy formed from the molten metal. Reoxidation products in the finished alloy formed due to exposure to oxygen, hydrogen, nitrogen or moisture can detrimentally effect the machinability, surface quality and other mechanical properties of the alloy.
Various methods or devices designed to prevent the contamination of molten metal streams by atmospheric gases during teeming operations have been developed. In one such method, a shroud of inert gas surrounds a molten metal stream to avoid contamination of the molten metal stream by atmospheric gases during teeming operations.
In particular, U.S. Pat. No. 4,840,297 issued to Weekley et al. describes a device for shrouding a stream of molten metal with an inert gas as it is being transferred from one vessel to another. The Weekley et al. reference describes the use of a shroud made from monolithic ceramic fiber in combination with a metal ware to maintain a shroud seal between a pouring ladle and a receptacle for molten metal. The shroud described in Weekley et al. appears to be a formed, rigid device. A metal ware assembly is used in conjunction with the ceramic fiber shroud to interconnect the ladle and receptacle vessel. The combination of the shroud and ware assembly provides a positive seal between the ladle and receptacle vessel. The shroud/ware assembly combination is then purged with an inert gas by way of a gas manifold which is positioned within the ware assembly. Devices which rely on rigid shrouding enclosures frequently require permanent and expensive mechanical modifications to both the ladle or receptacle vessel. Expensive, cumbersome support devices are often required to prevent the transfer of the weight of the metal laden ladle to the trumpet stand when the two are interconnected by a rigid shroud. Such support devices and mechanical modifications represent a significant cost and may preclude the use of the device in existing facilities. Furthermore, because these devices are made from rigid materials, they are not adjustable to accommodate a variation in distance between the ladle and the receptacle.
Another method known to be used for shrouding the transfer of molten metal from a ladle to a receptacle vessel involves the use of an asbestos curtain loosely draped over the opening of the receptacle vessel to provide a physical enclosure for a flow of inert gas around the molten metal stream. Hashio, M. et al., "Improvement of Cleanliness in Continuously Cast Slabs at Kashima Steel Works, "2nd Process Technology Conference on Continuous Casting of Steel (Chicago, Ill.) ISS-AIME, 1981, pp. 180-187. Curtain enclosures do not maintain a positive seal between the shroud and the receptacle vessel.
Other methods and devices have also been used to provide atmospheric shrouds around transfer streams of molten metal. For example, a reference by Vonesh et al., entitled "Inert Gas Shrouding of Molten Metal Streams" presented at the 1986 AISE Spring Conference in Philadelphia, Pennsylvania, describes a method and apparatus which provides a carefully controlled flow of inert gases around a molten metal stream without a physical enclosure to contain the inert gases. Methods and devices such as described in the Vonesh et al. reference which do not include a physical enclosure suffer from the drawback that they require large amounts of the inert gas to provide the shrouding effect.
Consequently, a need exists for a shrouding method and device which will effectively shroud a molten metal stream as it is transferred from a ladle to a receptacle, without the need for significant mechanical modifications to either the ladle or receptacle. There is also a need for a shrouding method and device which can be used over a range of distances between the ladle and receptacle. Finally, there is a need for a shrouding method which minimizes the amount of inert gas used.